With an advance in high density and miniaturization of recent printed circuit boards, there is a demand for a method capable of producing at high speed an ultrathin copper-clad laminate whose main copper foil has a thickness of 10 .mu.m or less.
Printed circuit boards or copper-clad laminates which are manufactured by the so-called transfer method are conventionally known. Examples of such boards or laminates are disclosed in Japanese Patent Publication No. 55-24141, Japanese Patent Publication No. 55-32239 (U.S. Pat. No. 4,053,370), Japanese Patent Publication No. 57-24080, Japanese Patent Publication No. 57-39318, Japanese Patent Disclosure No. 60-147192, U.S. Pat. No. 4,715,116, etc.
In a process (hereinafter referred to as belt transfer process) for producing printed circuit boards disclosed in Japanese Patent Publication No. 55-24141, Japanese Patent Publication No. 55-32239 (U.S. Pat. No. 4,053,370), Japanese Patent Publication No. 57-24080, and Japanese Patent Publication No. 57-39318, a printed circuit board is produced in the following manner. A thin, electrically conductive metal belt, which slides on the outer peripheral surface of a metallic rotating drum or a cathode portion of a horizontal plating apparatus, is used as a cathode. The metal belt is transported while being kept at a predetermined distance from an insoluble anode. A plating solution is supplied forcibly between the metal belt and the anode at high speed, thereby electrolytically forming a copper circuit on the surface of the metal belt. After an insulating substrate having a bonding agent previously applied thereto is adhered to the copper circuit, the insulating substrate and the copper circuit are separated from the metal belt. Thus, the printed circuit board is completed. Adapted for high-speed plating, the belt transfer process has the advantage over conventional methods that it permits very fast deposition of copper and continuous production of the printed circuit boards. However, the belt transfer process is associated with the following drawbacks. During a separation step in which the insulating substrate having the copper circuit transferred thereto is separated from the metal belt, part of the copper circuit may not be able to be transferred to the insulating substrate, due to the difference between the strength of adhesion between the copper circuit and the metal belt surface and that between the insulating substrate and the surface of the copper circuit, and other causes. For the same reason, the copper circuit may swing or be deformed during the transfer and separation steps, thereby causing defects such as wrinkling, breakage, bruises, cracks, etc.
The belt transfer process is usually employed to produce printed circuit boards. When applying it to the manufacture of copper-clad laminates, the following problems arise.
In the belt transfer process, a long metal belt is generally used as the electrically conductive substrate, but in the case of manufacturing copper-clad laminates, it is essential that the width (510 to 1,020 mm at the least) of the copper-clad laminate produced is greater than that of a printed wiring to be formed. Further, when employing the belt transfer method for transferring copper circuits or copper foils and also using the so-called reel-to-reel method disclosed in Japanese Patent Publication No. 57-24080, it is desirable to use a relatively thin metal belt with a thickness of the order of 0.2 to 0.8 mm, in view of the residual stress of the belt wound around a roll.
However, a metal belt which is wide and thin and also uniform in structure is hardly available, and if available, such a belt will be very expensive. Further, in general, a metal belt has a lateral swerve of about 5 mm per the length of 20 m, or its side edges are more or less corrugated. Therefore, the belt zigzags as it is run during the belt transfer step, so that portions of the copper foil at the edges of the belt are scraped off by a squeegee bar, thus entailing non-uniform thickness of electrodeposition, as well as machine troubles of the line due to short circuit or flaws in the belt caused by the copper powder scraped off from the copper foil. To eliminate these problems, an auxiliary mechanism such as sensors or the like may be incorporated into the machine line. This, however, results in a remarkable increase of the line cost and a hindrance to improvement of productivity, and accordingly is not a desirable measure from the industrial standpoint.
Particularly when manufacturing ultrathin copper-clad laminates by the belt transfer process, the copper foil is liable to swing or be deformed during the transfer and separation steps, thus causing defects such as wrinkling, breakage, bruises, cracks, etc.
Thus, the conventional belt transfer process cannot be used to produce a copper-clad laminate having a width of 510 to 1,020 mm at least, without arising the various problems.
In the so-called reel-to-reel method disclosed in Japanese Patent Publication No. 57-24080, for example, a metal belt of stainless steel on a reel is wound therefrom by another reel. With this arrangement, the surface of the stainless-steel plate is liable to suffer flaws, soiling, or other damages. If work is suspended to remove soil or flaws on the plate, the formation of the copper foil, in its turn, is spoiled. Thus, according to the reel-to-reel method, the work (line) cannot readily be suspended even when the stainless-steel surface suffers soiling, flaws, or other damages. This results in an increase in fraction defective, reduction in working efficiency, etc.
If stainless steel is used for the metal belt, moreover, inevitable physical or electrochemical defects, such as pores, exist on the surface of the metal belt. According to the belt transfer process, the copper circuit or copper foil is electrolytically precipitated in a direct manner on the surface of the metal belt having such defects, and therefore pinholes tend to be produced in the deposited copper. Particularly when the thickness of deposited copper is 18 .mu.m or less, the defects of the metal belt surface are reflected directly upon the production of pinholes. Therefore, in manufacturing a circuit board including copper circuits of 130 .mu.m wide and having a fine circuit pattern with a width of 130 .mu.m or less, critical defects in quality can be caused, such as deficiency or disconnection of the copper circuits. The problem is particularly serious for high-density conductor circuit boards with a copper circuit width of 100 .mu.m or less and a circuit interval of 100 .mu.m or less.
When the film thickness is 18 .mu.m or more, the number of pinholes is reduced, and when the copper foil thickness is 35 .mu.m, pinholes of this type make no hindrance to the formation of circuits.
For the transfer methods described above, it is essential that the copper foil obtained is free from pinholes and, if some pinholes exist, the number and size thereof are so small that no defects are caused during the formation of a fine pattern as described above.
The occurrence of pinholes generally depends on the surface conditions of the electrically conductive substrate used and the electroplating conditions inclusive of the plating solution used.
For example, an electrically conductive substrates used for the transfer method is produced by rolling an ingot of an electrically conductive material, such as stainless steel, nickel, or copper, into a plate. The electrically conductive substrate produced in this manner, however, is not entirely free from defects, that is, even if a grinding or polishing treatment is applied physically or chemically to the substrate surface, nonmetallic inclusions and intermetallic compounds, mixed in the substrate during the fusion and machining processes, and pores such as oil pits cannot be completely removed from the substrate. Therefore, when metal is electrodeposited on the conductive substrate, the defects of the substrate surface are duplicated onto the electrodeposited metal layer, since the duplication precision of electrodeposited metal is approximately 0.05 .mu.m. Particularly, if undercut pores exist in the substrate surface, copper, for example, enters the pores during the electrodeposition. Accordingly, when the conductive substrate is separated from the insulating substrate after transferring and laminating the electrodeposited copper onto the insulating substrate, part of the electrodeposited copper is torn off at the pores and remain on the conductive substrate, thus producing pinholes corresponding in position to the pores. This situation is observed quite frequently in the manufacture of copper-clad laminates with a thin copper foil of less than 18 .mu.m thick.
A method (hereinafter referred to as conventional transfer method) of producing conductor circuit boards disclosed in Japanese Patent Disclosure No. 60-147192, mentioned above, comprises a step of forming a thin metal layer on the surface of an electrically conductive substrate, a step of roughening the surface of the thin metal layer, a step of forming a resist mask conforming to a printed wiring to be formed, on the thin metal layer, and a step of plating copper on those portions of the substrate on which the resist mask is not formed, to thereby form desired copper circuits. Then, in a subsequent transfer step, the copper circuits, the resist mask and the thin metal layer are transferred together to an insulating substrate, followed by the removal of the thin metal layer by etching and of the resist mask by etching or by a solvent suited for the resist mask.
This conventional transfer method has the advantage over the aforementioned belt transfer process in that the thin metal layer ensures the transfer of the copper circuits and the resist mask to the insulating substrate.
However, the conventional transfer method is concerned with nothing but the manufacture of printed circuit boards, and is not a method for producing wide, copper-clad laminates, in particular, ultrathin copper-clad laminates, at high speed.
There has been proposed another transfer method by U.S. Pat. No. 4,715,116, which comprises a step of electrodepositing a first copper layer, which suffers few pinholes, directly on the polished surface of a smooth metal plate, a step of forming a second copper layer by using a different plating solution, a step of roughening the surface of the second copper layer, a step of pressure-bonding an insulating substrate to the roughened surface of the second copper layer with heat, and a step of detaching the copper layers at the polished surface of the metal plate. In this method, as is described in the embodiment, the first thin copper layer is electrodeposited on the surface of the metal plate (electrically conductive substrate) by using a plating solution of copper cyanide or copper pyrophosphate which produces few pinholes. Then, after copper is electrodeposited on the first copper layer to a predetermined thickness by using a copper sulfate bath, a further copper layer is electrodeposited on the copper layer, using a plating solution different from that used for the second copper layer, followed by the roughening of the copper layer. There is a statement in the specification that the first and second layers are as a whole substantially pinhole-free.
Here, the problem of pinholes will be considered from the viewpoint of electroplating conditions. In the case of using a copper pyrophosphate solution or copper prussiate solution as the plating solution on the premise that the electrically conductive substrate is satisfactorily polished and the electrodeposition step is carried out under ideal conditions, the supply of copper ions to the conductive substrate is controlled by a chelating agent contained in the plating solution. Thus, a copper film consisting of relatively small crystalline grains can be uniformly electrodeposited, and pinholes of the electrodeposited copper film can advantageously be reduced. On the other hand, in the case of using the above plating solution but under the condition that high-speed plating capable of copper deposition at 15 .mu.m/min or more is carried out with use of an insoluble anode to achieve high productivity, oxygen is generated during the electroplating and thus the pH of the plating bath varies, so that the insoluble anode becomes inoperable and no longer functions as the anode. If a soluble anode is used instead, the dissipation of the anode is so drastic during the electroplating that the distance between the cathode (conductive substrate) and the anode cannot be maintained at a constant value. Uniformity in thickness of the electrodeposited copper film is greatly influenced by the precision of the distance between the electrodes, and accordingly, a copper film with a uniform thickness cannot be produced if the interelectrode distance is not maintained at a constant value. Thus, with the above-mentioned plating solutions, it is very difficult to carry out a satisfactory high-speed plating and reduction of the productivity during the formation of a copper foil is unavoidable.
In the method disclosed in the aforementioned U.S. Pat. No. 4,715,116, the current density for the first layer is 1 to 7 A/dm.sup.2 (in the case of a copper cyanide solution) or 2.2 to 4.3 A/dm.sup.2 (in the case of a copper pyrophosphate solution). Taking the material of the anode used for forming the first layer into account, and also considering the fact that the specification discloses no solution flow speed, the first layer at least is not formed by the high-speed plating method. It is generally known to one of ordinary skill in this technical field that, when the aforementioned plating solution, i.e., copper pyrophosphate solution or copper cyanide solution, is used, electrodeposition effected therein is controlled by the chelating action, and as a result, production of pinholes is remarkably reduced. In the above method, the first copper layer is formed to prevent pinholes, and then the second copper layer is formed on the first copper layer to obtain a copper foil with a desired thickness. To obtain a pinhole-free copper foil with this method, it is essential to carry out a plating process using a copper pyrophosphate solution, etc., which is slow in electrodeposition speed, to form the first copper layer. Moreover, the first and second layers are formed by different plating processes, and accordingly additional plating tank and plating solution must be prepared, making the method not very effective.
The high-speed plating is described in detail in the aforementioned Japanese Patent Publication No. 55-32239 (U.S. Pat. No. 4,053,370). This publication, however, deals with the high-speed plating only from the viewpoint of improving productivity; it does not refer to the problem of pinholes as described above. Further, the process disclosed is for the manufacture of printed circuit boards.
In an alternative prior art process for producing a copper-clad laminate, copper is deposited, by electroplating, to the surface of a carrier made of an aluminum foil 40 to 60 .mu.m thick, thus forming a copper foil with a thickness of 5 to 10 .mu.m. Then, an insulating substrate is bonded to the surface of the copper foil for lamination, and the aluminum carrier is removed chemically by means of a chemical agent, or is separated mechanically. In another prior art process, a copper ingot, for example, is rolled into a copper foil with a thickness of about 3 .mu.m by a multistage rolling mill, and the copper foil is pressure-bonded to an insulating substrate.
In the former process, however, a complicated step is required for the removal of the aluminum-foil carrier, and in addition, the aluminum foil is partly left unremoved even after the foil removal step, due to burrs created during a through hole-forming step. In the latter process, on the other hand, the rolling method for the manufacture of the copper foil is used in place of the plating method in the aforementioned belt transfer process. In this case, if copper is rolled into a foil with a thickness of 3 to 5 .mu.m, the copper foil obtained inevitably has a size of 100 mm square or less. Thus, copper-clad laminates with a greater width cannot be produced with this method. Further, handling such a thin copper foil alone may result in wrinkling, soiling, etc. of same and accordingly in high cost and low productivity.